Projection exposure systems are used in semiconductor fabrication to transfer a circuit pattern formed on the surface of a mask (the objective plane), through imaging optics, onto a substrate, e.g. a wafer coated with resist. Exposing this resist with the projected image of the circuit pattern forms a latent image of the circuit pattern in the resist.
The major advantage of electron beam exposure, in which an electron beam is used to form this pattern instead of a projected light beam, is that the size of the electron beam can be reduced to just a few Angstroms, if desired, to form fine patterns as small as 1 .mu.m or less.
One of the electron beam exposure techniques used in the past was the direct write technique. In the direct write technique, fine patterns are formed by scanning a converged electron beam within very minute areas that are actually smaller than the line width of the pattern to be written.
The problem with the direct write electron beam exposure technique is that it is too slow to be used where high throughput is required. Another technique, however, is electron beam projection exposure, in which a circuit pattern formed on a mask is projection-transferred to a substrate. Electron beam projection exposure is capable of higher throughput than can be obtained with conventional direct write techniques.
A simplified schematic representation of a prior art electron beam projection exposure apparatus is shown in FIG. 1 which comprises:
a mask (2) stage, PA1 a conventional illumination optical system (1) which consists of an electron gun and optics (not shown) for illuminating the mask (2) with an electron beam (7) emitted from the electron gun and PA1 a projection-imaging optical system (3) for projecting an electron beam (7a) which is transmitted by or passed through the mask (2) onto a substrate (4). PA1 an illumination optical system for generating an electron beam, PA1 a mask; PA1 a mask stage for aligning and scanning said mask relative to said electron beam; PA1 a substrate; PA1 a projection-imaging optical system in alignment with said mask for transferring the image of a pattern formed upon said mask by said electron beam to said substrate; PA1 a substrate stage for said substrate; and PA1 at least one filter located in the proximity of said mask for filtering dust and for preventing dust particles from inhibiting the passage of the electron beam to said substrate.
The electron beam 7, emitted from the illumination optical system 1, illuminates the mask 2, and the electron beam 7a, transmitted by or passed through the mask 2, passes through the projection-imaging optical system 3 forming electron beam 7b which illuminates the substrate 4. The substrate 4 might be, for example, a resist-coated silicon wafer. The electron beam 7b incident upon this substrate 4 modifies the resist, leaving it either more soluble (positive resist) or less soluble (negative resist) in the developer. Stated otherwise, the projection-imaging optical system 3 projects a reduced image of the circuit pattern of the mask 2 onto the substrate 4, which results in a fine circuit pattern being exposed in the resist on the substrate 4.
However, due to optical constraints of the projection-imaging optical system 3, high resolution can be obtained only in a small area (e.g. approximately 1 mm.sup.2). Since the size of a semiconductor chip is on the order of 20 mm.sup.2, it is not possible to expose the entire chip at once. Therefore, in a conventional prior art electron beam projection exposure systems in order to expose the desired area of the mask, the electron beam 7 is directed at a selected portion of the mask (pattern portion) and is also scanned as required to expose the desired pattern portion area.
In addition to scanning the electron beam, the mask may also be scanned. To do this, the mask may be scanned by a mask stage scanning mechanism. This may be accomplished, for example, by scanning the electron beam along one axis and the mask along another intersecting axis, thus providing a large exposure field.
The pattern portion of a mask for electron beam projection exposure might consist, for example, of a membrane. Since such membranes have low mechanical strength, they are usually supported at the outer edges by retention members.
FIG. 2 is an enlarged view of a pattern portion 21 of a mask 2. The pattern portion 21 might be formed, for example, by making multiple through-holes 8 in the mask within the designated portion 21 in the shape of a pattern. When this has been done, one electron beam ray 14 which is directed at a through-hole 8 in the pattern portion 21 will pass through the through hole 8, whereas another electron beam ray 15 which is not directed at a through hole 8 i.e., is directed at another portion on the pattern portion 21 of the mask will either be scattered, or absorbed.
Electron beams that pass through the through-holes 8 are formed into an image at the substrate 4 of the mask circuit pattern by the projection-imaging optical system 3. If there is a minute dust particle 22 on the surface of the membrane such as a minute particle of dust 22 located at a through-hole 8 it may serve to block an electron beam ray 16 directed toward that through-hole and end up being absorbed or scattered by the dust particle 22. When this occurs, the pattern will often not be faithfully transferred to the substrate in areas where through-holes are blocked by dust. For this reason, such prior electron beam projection exposure systems have a problem in that circuit patterns from exposures using such systems tend to have many defects.
Although there are mask cleaning methods to mitigate this problem, most prior cleaning methods involve cleaning the mask before it is mounted in the electron beam projection exposure system and dust which collects on the mask between cleaning and mounting still result in defects in the patterns formed by the system.
Even when the mask is cleaned just prior to loading it in the system there is still a problem in that dust particles inside the exposure system adhere to the mask, and as the exposure time increases, the defect density also increases.
It is an objective of the present invention to provide an electron beam projection exposure apparatus in which accumulation of dust particles adhering to the mask can be prevented, thus reducing defects in circuit patterns formed on substrates through exposures made with the mask.